Solution
ExpressCards come in two form factors differentiated by the card widths, 34 mm and 54 mm. Although the width of the card is different, both cards use the same 26-pin contact connector. All ExpressCard/54 slots will except either ExpressCard/54 and ExpressCard/34 cards. Only ExpressCard/34 cards will fit an ExpressCard/34 slot.
NI products such as the ExpressCard-GPIB or the ExpressCard MXI are 34 mm cards and are compatible with both the 34 mm or 54 mm slots.